Polyamory is a subsection of the ethical non-monogamy umbrella. This umbrella is anywhere from large to massive, depending whose definition you use. At the most consensus definition, it's something like "Any relationship structure in which all partners are aware and consenting to some form of non-monogamy, be it sexual, emotional, or both." I know it's cheating to use part of the word in its definition, so we can extend it out to "any sexual, deeply emotional, or romantic relationship that allows for a structure in which all partners are aware that any individual may have more than one of these relationships, on any or all of these parameters, by agreement of all parties." It's even more of a mouthful then, but also starts asking us more questions.
By that second definition, is having a best friend you call to cry with or at when something happens as well as your romantic/sexual/emotional (ie traditional relationship escalator programming partner) with the knowledge of your escalator partner a form of ethical non-monogamy? I think most people would say no, just that that's a partner who is secure in your partnership and a relationship that is healthy but within the monogamous world, unless one of you is romantic or sexual with that person, or that person is the first person you go to with all your emotional needs. But it almost certainly would include queer-platonic relationships negotiated to only meet cuddling or handholding and emotional needs that are difficult to externally understand, or a relationship anarchist deconstruction of labels that says that emotional relationship is clearly deeply important to you so you prioritize it, regardless of societal assumed label.
Also, by that second definition, a purely romantic relationship, which I think would look different for any two people, would be fair game as an ENM relationship, and a purely sexual relationship, like early stages of swinging for many couples or continuing stages for some, or a combination of the two, like later stages of swinging for some swingers, would all be ethical non-monogamy. So would a friend with benefits, or, as a friend who helped me get myself kicked off apple podcasts for forgetting to mark the hour of content explicit for the one sentence called it, agreeing to fuck next Tuesday. They're all negotiated between consenting adults, they're all open about more than two people being involved, and hopefully in the case of the sexual ones about the sexual risks involved.
The traditional definitions of polyamory within that umbrella are essentially the middle of the spectrum, the ones where romance are the focus are the "point" and sex- or emotion-focused or sex- or emotion-included relationships are a "bonus" or "incidental" or something one partner at the end of a polycule does routinely but not everyone. It's why so many of the the people I speak to for the Ready for Polyamory podcast identify as former polyamoryists and current ethical nonmonogamists- they're saturated on romantic relationships but they're open to sexual and closely emotional in a friendly way relationships without either riding the relationship escalator or being deeply romantic. About half of the people I spoke to identify that way; I do the opposite -- I was ethically nonmonogamous without being specifically polyamorous in focus for years and now think I will be seeking romantic relationships in particular for quite some time.